18. the Geology of Peter I Island

18. THE GEOLOGY OF PETER I ISLAND

Thomas W. Bastien, Ernest K. Lehmann & Associates, Inc., Minneapolis, Minnesota and Campbell Craddock, University of Wisconsin, Madison, Wisconsin

INTRODUCTION The U.S.S. Burton Island, an icebreaker, landed par- ties both by boat and helicopter on the afternoon of 29 Peter I Island is located in the southeastern Pacific February 1960. Craddock was a member of a party Ocean at 68°45'S, 90°40'W, approximately 240 n.mi. which landed by boat on the shore of Norvegia Bay off the Eights Coast of West Antarctica (Figure 1). Ris- north of Cape Ingrid (Figure 2); approximately 2.5 hr ing from the upper continental rise, it is one of the few were available for geologic observation and collection. truly oceanic islands in the southeast quadrant of the Echo-sounder profiles were obtained by the ship, and Pacific Ocean. Few people have visited the island, and the shoreline was mapped by radar. The data and little is known of its geology. specimens collected on this visit form the basis for the Peter I Island was discovered by Captain Thaddeus present report. (or Fabian Gotliev) von Bellingshausen the afternoon The Russian ship Ob carried out oceanographic of 10 January 1821, while commanding the Russian studies near the island during the next month (Dubinin, sloops Vostok and Mirny (U.S. Navy Hydrographic Of- 1960). In January 1964, the U.S. oceanographic ship fice, 1960); his approach from the west was stopped by Eltanin made an approach to Peter I Island (Antarctic pack ice about 14 mi from the island. This discovery Report, 1964). During the 1970-1971 austral summer, was the first sighting of land south of the Antarctic Cir- an Argentine expedition conducted biological studies at cle (Debenham, 1945). On 14 January 1910, the the island. Pourquoi-Pasi, commanded by Dr. Jean Charcot, sailed Landings have been made at three locations on Peter within 3 mi of the island but also encountered pack I Island. Two of the locations are at Sandefjord and ice; this second sighting of the island was the first con- Norvegia bays, which are separated by the rock firmation of its existence. The Norwegian vessel Odd I promontory of Cape Ingrid (Figures 2, 3) and which circumnavigated the island in January 1927; they were have the largest areas of exposed rock accessible from unable to land, but dredge hauls provided specimens for the sea. The two landings by Nils Larsen, the 1948 land- the petrographic study of Broch (1927). In February ing from the Burton Island, and the 1956 Chilean land- 1929, Nils Larsen, captain of the Norvegia, supervised ing all took place in Sandefjord Bay. There are only sounding, charting, and dredging of the sea around brief descriptions of geologic observations made during Peter I Island; on 2 February a party made the first these landings. Holtedahl (1929, p. 83-88) mentions landing on the island and established a provision depot "richly olivine-bearing, very fine-grained rock" for a future expedition. During February of 1931 collected from the southeast corner of Sandefjord Bay. Larsen attempted to make another landing on the In 1960 the party from the Burton Island landed in island, but pack ice prevented the Norvegia from ap- Norvegia Bay, and Craddock was able to work briefly proaching closer than 36 mi. A month earlier the along the entire 50-ft-wide beach from Cape Ingrid English Discovery II also had been turned back by pack northward to Nils Larsen Glacier. The third location is ice 20 mi from the island. near the northeast corner of the island, where a The United States ships Bear and North Star circum- helicopter from the Burton Island landed in 1960. A navigated Peter I Island at a distance of 4 mi in two-man party was left there for several hours, but it February 1941. During mid-February 1948, the was unable to reach rock because of crevasses. Norwegian Brategg expedition, with Nils Larsen again the captain, landed the second party on the island; for 3 GEOMORPHOLOGY days they collected bird specimens and carried out Peter I Island is a glacier-capped volcanic island ap- geological investigations which "were the most in- proximately 6 mi wide and 13 mi long (Figure 4). The teresting [investigations] and could have afforded much highest point, according to the 1960 survey, is about new information, if we would have been able to com- 5750 ft above sea level (U.S. Navy Hydrographic Office plete them" (Holderson, 1948). This party was Chart 6713). The east and west sides of the island are evacuated 13 February because of encroaching pack marked by precipitous cliffs interrupted by several ice. Two days later, the U.S.S. Burton Island and U.S.S. glaciers which project into the sea. The principal rock Edisto arrived and put a small party ashore for an hour outcrops occur along these cliffs and along the gentler (Commander Task Force 39, Annex IA, p. 18). The sloping north end of the island. The southern third of Chilean Naval vessel Baquedano landed a party in the island is a gently inclined ice surface with isolated January 1956, at Sandefjord Bay (Thomas, 1957); no small rock outcrops; the ice at the seaward termination reports of their observations have been found. of this slope is probably grounded. Exposed rock is es- Helicopters launched from the USCGC Eastwind timated to form less than 5% of the island's surface. photographed the island in January 1957, but no land- The summit of the island is completely covered by ing was attempted (Henry Dater, personal communica- snow and ice, but a circular depression roughly 300 ft in tion, 1964). diameter, surrounded by an indistinctly defined rim ris-

341 McNAMARA s\ v [2~| ICE SHELF Approximate boundary •~•η ffi• r;::r\ \ ( 'v_^ Bathymetric data limited /'^}{• —'ISLAND : )\^ ) 250 Fathom Contour Interval

.> \ \

JONES MOUNTAINS

Figure 1. Bathymetric map of the Bellingshausen Sea (after U. S. Navy Hydrographic Office Chart 6638). GEOLOGY OF PETER I ISLAND

Figure 2. Aerial view of west side of Peter I Island showing Nils Larsen Glacier (left), Norvegia Bay, and Cape Ingrid (right). U. S. Navy photo, 29 February I960. ing about 100 ft above the depression, suggests the the plateau-like ice sheets which border much of the presence of a volcanic crater beneath a relatively thin island. Crevasses are abundant in the valley glaciers, ice cap. No signs of recent volcanic activity were ob- but they are important only locally in the plateau areas. served, and the seeming lack of tephra layers in the Numerous new icebergs (Figure 3) and several numerous ice cliffs suggests a considerable period of in- avalanches observed during the 2-day visit by the Bur- activity; none of the previous expeditions reported any ton Island in 1960 suggest fairly rapid ice movement. volcanic activity. A single K-Ar whole-rock age of 12.5 The snow accumulation rate in the Eights Coast area to ±1.5 m.y. (Table 1) was obtained on specimen PI-5, the south is quite high (Shimizu, 1964), and Peter I olivine basalt collected from a flow at Cape Ingrid; this Island, surrounded by or close to open water for a short apparent age also suggests the possibility of long inac- period each austral summer, may have a similarly high tivity. On the other hand, the elevation of 5750 ft above or even higher accumulation rate. sea level obtained for the highest peak in 1960 is much Holtedahl (1929) discusses the geomorphic features greater than the readings of about 4000 ft obtained by of the island, based on photographs taken by the Nor- von Bellingshausen in 1821. vegia expedition. It is his opinion that the precipitous There are at least four large valley glaciers which ter- east side of the island was formed by intensive wave minate as ice tongues floating in the sea (Figures 2, 5). erosion, instigated by a postulated prevailing easterly The fan-like terminus of the largest glacier is about 3 mi wind, and that the west side was eroded by both waves wide. Ice cliffs of approximately 80-100 ft in height oc- and glacial action. He also infers extensive marine cur at the foot of these glaciers and along the edges of planation, both at present and during an earlier high

343 T. W. BASTIEN, C. CRADDOCK

Figure 3. Aerial view to south along west side of Peter I Island showing Cape Eva (foreground), Nils Larsen Glacier, and Cape Ingrid (distance). U. S. Navy photo, 29 February I960. stand of sea level. Definition of the underwater and indicates that with continued erosion and sub- morphology of the island during the 1960 survey con- sidence Peter I Island would become a typical guyot. firms the existence of a well-developed marine plat- Projection of the 15° submarine slope to form a form. volcanic cone would place the restored summit at ap- The base of Peter I Island lies approximately 2000 proximately 7000 ft above sea level (Figure 4). fathoms below sea level and is roughly circular in form, However, the low apparent dips of the exposed volcanic with a diameter greater than 40 mi (Figure 4). The strata suggest that the volcano may have been more 2000-fathom base level extends about 45 n.mi. south- shield-like and that the summit may never have been eastward to the base of the continental slope. The sub- much higher than at present. The possibility exists that marine slope around the island averages about 10° to an increase in the amount of more viscous 15°, except for the distinct subhorizontal platform 15- trachyandesitic magma during the later stages of activi- 20 mi in diameter and less than 200 fathoms deep which ty may have resulted in steeper original slopes on the surrounds the island. The width of this platform east upper part of the island. and west of the island is somewhat greater than north Cape Ingrid represents a unique land form on the and south, as expected from Holtedahl's concept of island. It is a rocky promontory which extends out areas of maximum erosion. It seems probable that the perpendicularly from the west coast (Figures 2, 3). Its platform cutting has taken place with sea level nearly present form can be partially attributed to differential the same as, or possibly lower than, at present. A com- erosion of volcanic strata which have been intruded by parison of an east-west profile of the island, using the a small resistant pluton. The cape forms the southern sea floor as a base, with other volcanic islands and boundary of Norvegia Bay and, along with Nils Larsen guyots of the Pacific (Figure 6) shows their similarities Glacier, protects the narrow beach of this bay. The

344 GEOLOGY OF PETER I ISLAND

90°

BATHYMETRIC MAP OF CONTROL POINT FROM FATHOGRAM PETER I ISLAND 68°- From Fαthogrαms by U.S. Navy survey February 28-29,1960 -68° 30' 30' Shoreline from U.S. Navy radar photography SOUNDINGS IN FATHOMS ELEVATION IN FEET

Ah . HB

69°- -69°

5 4 3 2 10 lOmiles

CONTOUR INTERVAL 200 FATHOMS

90c

SEA LEVEL

WEST-EAST SECTION HORIZONTAL SCALE EQUALS VERTICAL DEPTHS IN FATHOMS Figure 4. Bathymetric map of Peter I Island area, based on U. S. Navy survey, 28-29 February 1960.

345 T. W. BASTIEN, C. CRADDOCK

TABLE 1 STRUCTURE Whole-Rock Analysis, Specimen PI-5 Peter I Island is a volcanic mountain composed of interbedded flows of basalt and more siliceous lavas, Material b a Ar /K 6 Specimen Analyzed %K 40 40 Age (10 yr) which range in thickness from 2 to at least 40 ft with irregular lateral variations. The proportions of dark PI-5 Olivine basalt, 1.035 0.00073 12.5 ±1.5 basalts and more siliceous rocks seem quite variable; whole-rock outcrops along the north coast appear from the air to be mainly basalt while at Cape Ingrid light-colored aAnalysis by Geochron Laboratories, Inc., Cambridge, Massachusetts. 10 10 rocks are an important part of the sequence. The Decay constants: λe = 0.585 X 10" /yr, λb = 4.72 X 10~ /yr. greatest observed dip of these volcanic rocks is about 5° "Radiogenic Ar Q. 4 seaward at Cape Ingrid. Strata on either side of the cape show gentle dips away from the cape, suggesting beach is composed of sand- to boulder-sized clasts of that there may have been an original headland resulting glacial ice and of the volcanic rocks which make up the from a subsidiary eruptive center or a series of overlap- adjacent cliffs (Figure 7). A 10-ft conglomerate bed of ping lava tongues from the main vent. The existence of well-rounded cobbles and boulders of ice and rocks oc- the cape may be largely due to this headland; the pres- curs at the north end of the beach (Figure 8). Some of ently exposed intrusive plug may have acted as a but- this material may be derived from the pack ice which tress only during the last stages of erosion. surrounds the island most of the year. Above the The dark gray basaltic lavas vary from dense to narrow beach rise steep rock cliffs, capped in turn by an highly vesicular; most vesicles are small and nearly active ice cliff (Figure 2). spherical, but some are distinctly elongate. A few sur-

Figure 5. Terminus of Nils Larsen Glacier, from the beach in Norvegia Bay, 29 February 1960. Height of ice cliff is estimated at 80 feet near the beach. C. Craddock photo.

346 GEOLOGY OF PETER I ISLAND

4r 2F 5 10 20 KM SEA LEVEL - ^^^-^^^ KM o["" ' ••H ••••••^― IE PETER 1 ISLAND W-E PROFILE

GUYOTS M GUYOTS MILLER GUYOT SYLVAN 1A GUYOT GULF ___^^^^^ of GUYOT •fc. PRATT ALASKA PACIFIC GUYOT 29° N 153° E GΔ -3 GUYOT ^^^• ••^

LAYSAN ISLAND ^• HAWAII GROUP NE-SW PROFILE ACROSS MAUNA LOA HAWAII

Figure 6. Profiles of some Pacific volcanic islands and guyots. faces have pahoehoe structure and ropy crusts (Figure canoes but with slightly greater slopes. Its submarine 9); no pillow structures were seen in any of the volcanic profile, as described in the previous section, has slopes rocks. Columnar jointing is common in the thicker which average 10° to 15°, but vary from 5° to 20°; basalt flows. Some basalt flows contain megascopic these compare with 4° to 6° for Hawaiian volcanoes olivine as crystals and granular inclusions. Some dis- (Rittman, 1962, p. 119, and our Figure 6). tinct gray flows are separated by highly vesicular basalt which is oxidized to a reddish-brown. At the base of the talus slope separating the rock cliff from the beach, just DREDGED ROCKS DESCRIBED BY BROCH north of Cape Ingrid, a 6-in., rounded, highly vesicular Broch (1927) has presented the only previous report basalt specimen was collected; this rock probably form- on rock specimens from Peter I Island. The 175 rocks ed as a bulbous squeeze-up (Macdonald, 1967). that he examined were dredged in 1927 from depths of 6 At Norvegia Bay light-colored trachyandesite flows to 7 fathoms off the west coast of the island near Cape are common and occur interbedded with the more Ingrid. He described the collection as poorly rounded abundant basalts. These rocks contain numerous and beach pebbles ranging between hazelnut- and twice fist- conspicuous gabbroid inclusions. Flow banding and size, and he identified three rock types: basalt, andesite, deformed vesicles are present in some flow units. and trachyandesite. In addition to the basalt and trachyandesite flows, Basalt specimens described by Broch are dense to the Norvegia Bay sequence also contains several brec- highly vesicular with hypocrystalline to holocrystalline cia beds of reworked older volcanic material. The porphyritic textures. Olivine is the principal pheno- largest clast observed is about 8 ft long. These beds are cryst, occurring as euhedral individuals up to 2 mm heterogeneous in both composition and texture; some long with some spinel inclusions; iddingsite rims are are massive, but some are stratified. One 8-ft bed con- common. Euhedral plagioclase crystals as large as 3 × tains a large angular block of trachyandesite with gab- 0.5 mm are the next most common phenocrysts; in- broid inclusions. dividuals with oscillatory zoning range from Amo to The stratified rocks have been intruded by both dikes An6o with prominent albite and pericline twinning. and small stocks. Many small basalt dikes cut the strata Augite phenocrysts occur in some of the basalt along Norvegia Bay; they are commonly less than 18 in. specimens; they are euhedral but corroded crystals up wide. Plug-like masses of basalt (?) were seen near Cape to 5 mm long, gray-brown with a tinge of violet, Eva, and other light gray intrusives occur at Cape lamellar twinned, and zoned with common hourglass Ingrid and at the southeast corner of Sandefjord Bay. structure. Glassy specimens contain only olivine as These intrusive bodies are not abundant and appear to phenocrysts. The matrix of the basalt is composed represent only a minor fraction of the island's total mainly of progressively zoned plagioclase with a max- rock volume. imum Anóo content, and augite which optically The gentle dips of the layers comprising Peter I resembles the phenocrysts. Olivine and metallic Island suggest that its form at maximum development minerals in roughly equal amounts are subsidiary com- was probably shield-like, resembling the Hawaiian vol- ponents, and apatite is a minor constituent. The matrix

347 T. W. BASTIEN, C. CRADDOCK

Figure 7. Clasts of glacial ice and volcanic rocks on the beach, Norvegia Bay, 29 February 1960. C. Craddock photo. crystals average about 0.05 to 0.1 mm in length. A dark The 11 trachyandesite specimens are light gray to red translucent matrix occurs in some fragments and pink holocrystalline porphyries with pilotaxitic to may be partly devitrified hyalopilitic rock. Calcite oc- trachytic matrices and many irregularly bounded pores. curs in minor amounts as vesicle fillings. Phenocrysts include (1) plagioclase, which is pro- Broch's 11 andesite specimens are aphanitic and gressively zoned from Amo to Anio, (2) basaltic horn- dense to partially vesicular with plagioclase pheno- blende which is strongly pleochroic and commonly crysts as long as 1 mm and lesser phenocrysts of olivine, decomposed to metallics and diopsidic augite, (3) basaltic hornblende, green diopsidic augite, and brown biotite similar in appearance and alteration to the basaltic augite. The plagioclase occurs as corroded idio- basaltic hornblende, (4) diopsidic augite which may be morphs zoned from An35 to An2o and as unzoned An.i2 the decomposition product of hornblende, and (5) crystals. Olivine of similar size is idiomorphic to cor- apatite containing needle-like inclusions oriented roded, and basaltic hornblende clouded with metallics parallel to the c axis which impart a pseudopleochro- occurs as somewhat rounded rectangular grains. Diop- ism to the blue-gray crystals. Plagioclase is the main sidic augite phenocrysts are slightly smaller, but matrix mineral; it is progressively zoned from Am? to similarly corroded; lamellar twinning is common. One Anio with some albite and Carlsbad twinning. Anortho- individual crystal of basaltic augite was seen. The clase may be present as rims and individual crystals. matrix is very fine grained and is predominantly albite- Diopsidic augite, pale green to brown with irregular twinned plagioclase. Brown augite with distinct hour- boundaries, is common. Irregular metallics are also glass structure is the next most common mineral, occur- common, and sphene and zircon occur as rare idio- ring as small (0.01 × 0.03 mm) prisms. Small metallic morphs. Apatite is present as small inclusions in plagio- grains are common, and minor apatite and rare zircon clase. A very small amount of a colorless, low-index are also present. material occurs in interstices and may be glass.

348 GEOLOGY OF PETER I ISLAND

Figure 8. Conglomerate bed with ice matrix and clasts of glacial ice and volcanic rock, north end of beach, Norvegia Bay, 29 February 1960. E. Doss photo.

ROCKS COLLECTED IN 1960 phenocrysts exceed 1.5 mm in length; they more com- ON THE SHORE OF NORVEGIA BAY monly range from 0.5 to 1.0 mm. Matrix crystals of all minerals average less than 0.3 mm, with plagioclase The rock collection obtained by Craddock on Peter I forming the largest crystals. Porphyritic textures are Island contains 29 specimens from the shore of pronounced only in specimens with intersertal matrices. Norvegia Bay between Cape Ingrid and Nils Larsen The mineralogical proportions of the basalt specimens Glacier. Sixteen specimens were collected in place, and show moderate variations, as summarized in Table 2. the remainder are beach cobbles and talus clasts. Thir- Olivine occurs as euhedral crystals that optically teen specimens are basalt, mainly from lava flows but have no apparent zoning. Their maximum length is 1.5 including one specimen from a dike; one specimen has mm, but they range down to matrix size; the largest an inclusion of trachyandesite, another an inclusion of crystals are embayed as a result of partial remelting or granular olivine. The remaining 16 specimens are chemical reaction with the magma. Small euhedral in- probably all trachyandesite, and 12 contain inclusions clusions of a spinel-type mineral occur as rare, random- of coarse-grained gabbroid rocks. ly oriented components in the olivine crystals. Altera- tion of olivine to an iddingsite-type mineral is common Basalts only in specimens PI-1, 9, and 15; the first of these is a The basaltic specimens are dense to scoriaceous and dense basalt, the others are oxidized scoriaceous dark gray to reddish-brown, the latter oxidized and specimens. generally more vesicular. Microscopic textures range Specimen PI-1 contains a cluster of olivine grains from hypocrystalline to holocrystalline, and from non- with allotriomorphic granular texture, uneven extinc- porphyritic to porphyritic with the matrices intersertal tion which may be related to translation laminae to intergranular felty to trachytic. Olivine is the most (Wilshire and Binns, 1961), and reaction rims of finer common phenocryst, followed by minor plagioclase granular olivine and clinopyroxene. Metallic grains and clinopyroxene; excluding the pyroxene, these min- form a zone of inclusions along the border of the erals have a seriate relationship with the matrix. Few cluster. These characteristics are similar to those found

349 T. W. BASTIEN, C. CRADDOCK

Figure 9. Ropy pahoehoe basalt flow, Norvegia Bay. C. Craddock photo.

350 GEOLOGY OF PETER I ISLAND

TABLE 2 TABLE 3 Modal Analyses of Peter I Island Basalts Anorthite Content of Plagioclase in Six Peter I Island Basalts Specimen No. Minerals Specimen PI- (volume %) PI-1 3 5 7 3 4 7 8 9 15 Olivine 17.2 11.4 9.8 9.8 Maximum An % of Individual Plagioclase 38.2 34.6 25.3 30.6 Crystals (by Microscope) Pyroxene matrix 30.0 34.4 41.6 38.4 50 51 47 51 55 53 phenocrysts 0.2 — _ 3.2 50 52 53 51 55 54 Metallics 14.4 19.6 23.4 18.0 52 53 54 52 55 53 56 58 in peridotite nodules in basalts and probably indicate a Electron Microprobe Analysis for Ca deep-seated origin for this inclusion. (range of An % within individual crystals) Plagioclase occurs principally as a groundmass mineral and only rarely as phenocrysts with oscillatory 51-55 39-51 zoning. Corroded remnants of large plagioclase crystals 54-57 41-42 49-56 42 are more common than phenocrysts; they may be 51-53 42-45 xenocrysts, perhaps related to the coarse-grained in- 54 56 clusions prevalent in the trachyandesites. Matrix 57-61 plagioclase crystals are generally euhedral to subhedral laths twinned according to the albite and Carlsbad laws. Their average size is about 0.2 × 0.05 mm, except clinopyroxene phenocrysts are corroded euhedra which for specimen PI-9 which contains crystals to 0.7 × 0.3 reach a maximum length of 1.6 mm. Twinning and mm with an average size of about 0.5 × 0.1 mm. progressive zoning are both common. The crystals are Table 3 lists anorthite contents of individual crystals colorless to pale yellow-green, with pale brown outer as determined microscopically by Carlsbad-albite ex- zones similar in color to the matrix pyroxene. Matrix tinction angles and Tobi's (1963) high-temperature crystals are euhedral to anhedral and generally less than plagioclase curves. Possible errors in turning angles and 0.1 mm in length. Optical determinations are very dif- in chart interpolation amount to ±3% An. Values in ficult, but both the phenocrysts and matrix crystals are this table represent maximum measured An contents probably in the diopside-augite group, with the pheno- of zoned crystals and early matrix crystals with twin- crysts possibly more diopsidic. ning developed well enough for measurement. Opaque metallic minerals form an important compo- Anorthite contents determined with electron micro- nent of the matrix; modal abundances range from 14.4 probe analysis of calcium by Bastien are also given in to 23.4 volume %. They are most commonly euhedral Table 3. The microprobe analyses represent spot deter- equant crystals less than 0.02 mm in diameter; sub- minations with a 2 µm diameter electron beam. There is hedral and anhedral crystals are not unusual, but ±2% An uncertainty related to the microprobe deter- skeletal forms are rare. The more poorly crystallized minations of calcium in the synthetic standards com- rocks such as PI-3, 5, and 7 contain larger amounts of pared with their chemical analyses. It should be empha- these opaque minerals, which may reflect a more rapid sized that the accuracy of the compositional values cooling that inhibited olivine and pyroxene from react- stated here relates to comparisons within the Peter I ing with the residual liquid, thereby leaving excess iron Island suite and not necessarily to absolute feldspar and titanium to form magnetite and ilmenite. Low compositions. Comparison of extinction angle and values for magnetite and ilmenite in the normative microprobe methods reveals generally close agreement mineral calculations (Table 5) also suggest incomplete in specimen PI-7, but a greater spread of results in PI-8. reaction. This reflects mainly the greater range in size of crystals Apatite occurs as a rare accessory mineral. It can analyzed with the microprobe in PI-8. The variation in only be found in specimens with a holocrystalline feldspar composition between the two specimens groundmass, as small rod-shaped crystals. Zircon is appears to be related to differences in cooling histories. even more rare and is found as small idiomorphs. A The greater size of feldspar crystals in PI-7 suggests a probable zeolite mineral occurs as a rare vesicle slower cooling rate, with feldspar crystallization coating. proceeding further before matrix clinopyroxene precipitation was initiated. Feldspars in PI-8 were in Trachyandesites greater competition for calcium with the clinopyroxene The trachyandesites are dense porphyritic aphanites, during crystallization than those of PI-7. pale pinkish-gray to light gray. Microscopic matrix tex- Clinopyroxene and plagioclase are the principal tures are trachytic to pilotaxitic, with 5% to 10% rock-forming minerals in the Peter I Island basalts; they irregularly shaped pores. Plagioclase and basaltic occur in roughly equal amounts and comprise more hornblende form the phenocrysts, which are as long as than 60% of the volume of the rock. Most of the 2.6 mm. The plagioclase is distinctly zoned, and twin- clinopyroxene is in the matrix, with only one specimen ning according to the albite, Carlsbad, and pericline (PI-7) containing as much as 3% phenocrysts. The laws is poor to well developed. Anhedral resorbed

351 T. W. BASTIEN, C. CRADDOCK

TABLE 4 Modal Mineral Estimates of Coarse-Grained Inclusions Specimen PI- Mineral 16 18 19 20 21 22 24 25 26 29a

Plagioclase -50 50 30 -50 -50 -40 -50 50 75 30-40 Ana 48 9 45,46 51,52, 45,46 69,70, 44,44 9 3142b 50 56 72 Diopsidic augite -

aAlbite-Carlsbad and Tobi's high-temperature curves. Electron microprobe analysis, zoned from An^^o m one grain. cOpacite (totals 25%). Includes opacite. crystals with euhedral overgrowths in the same spar; this assignment is supported by the chemical crystallographic orientation are common. Outer rims of analyses (Table 5). alkali feldspar are present on all the feldspar pheno- The matrix clinopyroxene is augite occurring as small crysts. Carlsbad-albite twin combinations are not well euhedral to anhedral, light gray crystals with maxi- enough developed for extinction angle measurements mum extinction angles of 43°; they form less than 5% of for composition determination. Maximum An content the total rock volume. A metallic mineral, probably from microprobe analysis of phenocrysts in specimen magnetite, also occurs as euhedral to anhedral equant PI-14 is An22 near the grain center for one crystal; three grains. Apatite is present as clear, rod-shaped euhedral other crystals had maxima of Ani5,15, and π. Outer zones crystals, occurring as inclusions in feldspar. Zircon is a are as low as Am. Or-Ab-An ratios were not deter- rare accessory mineral. A clear, low-index, isotropic mined. One small matrix grain shows zoning from Aniβ substance occurs interstitially and may be glass. to An2. Specimen PI-19 contains an embayed phenocryst zoned from Am? to An22 and a matrix grain zoned Inclusions in the Trachyandesites from An2β to An26. Small euhedral apatite inclusions in Coarse-grained, hypidiomorphic, granular gabbroid random orientations are common. inclusions occur in many of the trachyandesite The basaltic hornblende phenocrysts are corroded specimens. The mineralogy of these inclusions is idiomorphs which are almost entirely replaced by similar, but the relative abundance of the minerals aggregates of metallic and clinopyroxene granules. The varies considerably. The mineral constituents, in order parts which are unreplaced show strong pleochroism of generally decreasing abundance, are: plagioclase, from pale yellow to deep red. These phenocrysts and diopsidic augite, low-iron hornblende, basaltic their replacement products comprise about 5% to 10% hornblende, titaniferous magnetite, apatite, olivine, of the rock volume. and biotite. In Table 4 the volume percentages of the The matrix of the trachyandesites is composed of minerals are indicated as well as the plagioclase com- feldspar (which forms 50% to 80% of the rock volume), positions, which are listed according to increasing augite, apatite, magnetite, and cryptocrystalline anorthite content if known. The data are semiquan- material. Textures range from orthophyric to trachytic; titative because the volume percentages are visual es- the feldspar crystals average about 0.2 to 0.1 mm in timates from coarse-grained (grain size as large as 6 length, and the ferromagnesian minerals approximately mm) rocks in a single thin section for each specimen. 0.05 mm. The feldspar is strongly zoned, both The plagioclase of the inclusions is anhedral and in progressively and cyclically, undulatory and patchy ex- grains as large as 6 mm. Carlsbad, albite, and pericline tinction is common, and twinning is poorly developed twinning are prevalent; zoning is principally pro- as simple Carlsbad types. Electron microprobe ex- gressive, and crystals in contact with trachyandesite amination reveals compositions as calcic as oligoclase. have apparent alkali feldspar rims. Compositions vary The index of refraction of much of the feldspar is less considerably between specimens (Table 4), but the max- than 1.54, indicating compositions less calcic than Anio; imum An contents of the individual grains in any one the amount of alkali feldspar and whether it is anortho- specimen are similar. Microprobe examination of PI-26 clase or orthoclase could not be determined. The classi- revealed zoning from Am2 near the center to An34 at the fication of rocks of this type depends greatly on the edge of one crystal. composition and relative abundance of the feldspars. Clinopyroxene, probably diopsidic augite, is present These specimens are provisionally classified as trachy- both as a primary mineral and as an alteration product andesites, with alkali feldspar > 1/3 <2/3 the total feld- of basaltic hornblende. Anhedral forms are most com-

352 GEOLOGY OF PETER I ISLAND

TABLE 5 Chemical Analyses and Norms of Six Rock Specimens from Peter I Island (in wt %)

B-la B-2a B-3b B4C PI-5d B-5e PI-14f

61.13 SiO2 46.99 47.64 48.88 48.76 48.46 62.31

A12O3 12.61 12.79 12.46 12.70 12.90 17.28 18.49 Fe2θ3 3.21 3.25 3.77 4.23 2.52 2.65 4.77 FeO 8.94 8.96 8.09 7.41 10.93 1.67 0.58 MgO 10.50 10.64 8.64 8.57 8.39 1.41 1.09 CaO 8.13 8.25 8.70 8.70 8.63 2.42 2.51

Na2O 2.61 2.64 3.08 3.04 2.42 6.72 6.31

K2O 1.09 1.10 1.55 1.53 1.21 3.58 3.10

TiO2 2.85 2.89 3.25 3.25 2.94 0.72 0.51 P2°5 0.50 0.51 0.59 0.43 0.29 0.25 MnO 0.16 0.18 0.15 0.14 0.17 0.05 0.01 + H2O 1.00 1.01 0.15 0.33 0.40 0.11 0.03 H2°~ 1.30 - 0.67 0.66 0.09 0.30 0.43 0 0 0 0 0.36 0 1.25 co2 BaO 0.04 0.05 0.06 0.06 0.06 Cl 0.09 0.09 0.14 0.08 0.34 s 0.02 0.02 0.01 0.02 0.03 Total 100.04 100.02 100.19 99.91 99.42 99.94 100.46

q _ _ _ _ 5.52 9.84 or 6.67 9.45 8.90 7.23 21.13 18.35 ab 22.01 25.15 25.68 20.44 53.97 53.45 an 19.74 16.12 16.68 20.57 8.06 2.78 c — — — — — 3.67 di 14.24 18.87 18.78 16.29 1.99 - ol 11.44 7.02 5.08 8.17 — - hy 13.56 9.44 10.38 18.23 2.70 2.70 il 5.47 6.23 6.23 5.62 3.48 0.91 mt 4.87 5.57 6.03 3.71 1.37 0.46 ap 1.34 1.34 1.01 - 0.67 0.67 hm - - — - 0.32 4.48 cc - — — 0.80 - 2.90 hi 0.12 0.23 0.12 - 0.59 - Total 99.46 99.42 98.89 101.06 99.80 100.21

dPorphyritic basalt (Broch, 1927). bLight basalt (Broch, 1927). cDark basalt (Broch, 1927). ^Olivine basalt, this paper. Analysis by Technical Service Labs, Toronto. eTrachyandesite (Broch, 1927) - benmorite (?). "Trachyandesite," this paper - trachyte (?). Analysis by Technical Service Labs, Toronto. mon, but euhedral crystals also occur. The crystals complete pseudomorphs. The basaltic hornblende is show slight optical zoning, and twins are uncommon. yellowish-brown with strong pleochroism and Clouds of fine linear inclusions occur in the central part birefringence. Euhedral forms are more prominent of many crystals. Oriented blebs of what appears to be among the basaltic hornblende crystals than among the another pyroxene may be the result of exsolution. low-iron hornblende. There are two amphiboles present in most speci- Metallic minerals occur as both primary crystals and mens. The most common is probably a low-iron horn- as replacement products of basaltic hornblende. The blende showing slight pleochroism from pale pink to primary metallics generally form larger individual pale green. Crystals are fresh appearing, in contrast to crystals as magnetite-ilmenite oriented and nonori- basaltic hornblende which is commonly altered to ented intergrowths, and as individual crystals of mag- granular aggregates of metallics and clinopyroxene. netite and ilmenite. The replacement metallic mineral is The degree of replacement ranges from narrow rims to apparently magnetite.

353 T. W. BASTIEN, C. CRADDOCK

Apatite is a ubiquitous accessory and forms euhedral Irvine and Baragar (1971) propose a chemical to subhedral crystals as long as 0.5 mm that are com- classification system for dividing volcanic rocks into monly included in the feldspar, pyroxene, and groups and series. In their system the Peter I Island amphiboles. The apatite crystals contain many ex- basalts fall in a border region between alkaline and sub- tremely fine rod-like inclusions which parallel the c axis alkaline rocks, some criteria favoring the former and and possibly cause the faint pleochroism from pale gray others the latter. If the basalts are considered sub- to pale blue. This form of apatite is not uncommon in alkaline, they clearly belong to the tholeiitic series alkalic rocks (Tyrell, 1945; Le Maitre, 1962). rather than the calcalkali series. In this system the Peter Olivine was observed in only four of the coarse- I Island basalts seem closest to olivine tholeiite with grained inclusions, and in amounts less than 5% in three potash enrichment, but they also bear a similarity to of them. The crystals are anhedral with an iddingsite- slightly potassic alkali olivine basalt. Specimens B-3 type alteration along fractures. In specimen PI-19 a and B-4 are nearly identical and approach the closest to large plagioclase crystal surrounds a number of small, hawaiite in composition. optically oriented olivine remnants. Their composition The chemistry of the Peter I Island volcanic rocks is undetermined, but a straight isogyre observed from can be compared with that of similar rocks from an optic axis interference figure indicates a high 2V and Hawaii (Macdonald and Katsura, 1962, 1964), Gough therefore a high forsterite content. Island (Le Maitre, 1962), and three volcanic districts in Biotite is the least abundant mineral, occurring in West Antarctica (Craddock et al., 1964; Anderson, only three specimens. It shows strong pleochroism from 1960; Fenner, 1938) in an alkali-silica plot (Figure 10). pale yellow to ruby red and occurs as small anhedral The five chemical analyses of the four Peter I Island crystals. basalts plot near the transitional rocks of the Hawaiian suite, and the overlap in the latter between the alkalic Summary and tholeiitic series is evident. This diagram suggests The 1927 and 1960 rock collections from Peter I similarities between the well-documented Hawaiian Island are petrographically similar; however, there are and the poorly sampled Peter I suites. It also shows that a few differences. Andesite, as described by Broch, does the Peter I Island basalts are near the alkalic-tholeiitic not appear to be present in the 1960 collection, which boundary, but that the more siliceous rocks are consists wholly of basalts and probable trachy- probably in the alkaline series. andesites. Only one small fragment of a coarse-grained The solidification index (SI) of Kuno et al. (1957) for inclusion occurs in the Broch collection, but these gab- the analyses in Table 5 are given in Table 6. Values in broid inclusions are prominent in the later collection. the range of 35 to 40 indicate probably limited frac- The dredging naturally produces a random sample, but tionation of primary magma; lower values indicate the collecting on the shore was selective and biased greater fractionation. In Hawaii the tholeiites common- toward the rocks bearing the gabbroid inclusions. ly have SI values between 30 and 40 while the alkalic rocks are 15 to 20 but with values as high as 40 (Kuno, PETROCHEMISTRY 1962). Although the Peter I Island basalts have high SI Chemical analyses and normative minerals for six values, the statistical evidence of Kuno does not es- specimens of volcanic rocks from Peter I Island are tablish a tholeiitic character. given in Table 5. Four of these analyses and norms are On the basis of petrographic study, specimens B-5 from Broch (1927), and two are new here and based on and PI-14 have been termed trachyandesites. Because the 1960 collection. Specimens B-2, B-3, and B-4 repre- definitions of the intermediate rocks vary and because sent Broch's basaltic rocks, and B-5 his trachyandesite; our knowledge of the mineralogy of these two speci- in our collection specimen PI-5 is similar to Broch's mens is incomplete, we have retained Broch's (1927) basalt and PI-14 to his trachyandesite. The available rock name in the interest of consistency. The AFM dia- chemical analyses show a distinctly bimodal grouping. gram of Kuno (1968) shows these two specimens to lie However, Broch did not have a specimen of his andesite in the trachyandesite field. analyzed, and no specimen in the 1960 collection As with the four basalt specimens, efforts to classify appears to be comparable to his andesite. these two trachyandesite specimens by the Irvine and On the basis of their modes, specimens B-2, B-3, B-4, Baragar (1971) systems produce cloudy results. Alkali- and PI-5 can be termed olivine basalts, but a more silica and AFM plots indicate these rocks belong to the definite classification is difficult. The presence of calcic alkaline series, but other diagrams suggest transitional clinopyroxene and the apparent lack of reaction to subalkaline affinities. If these rocks are considered between olivine and pyroxene suggest that these rocks subalkaline, this system classifies B-5 as dacite and PI- are alkali basalts rather than tholeiitic basalts (Wilkin- 14 as rhyolite; but these names do not fit well with son, 1967). The chemical compositions of the four either the chemistry or known mineralogy and must be basalt specimens allow some refinement of their classi- rejected. However, if the specimens are treated as fication. The low alumina values indicate that these alkaline rocks, the system classifies B-5 as a probable rocks are not the high-alumina basalts of Kuno (1968), benmorite and PI-14 as a trachyte. Both rocks plot near but the distinction between alkalic and tholeiitic basalts the trachyte field on the alkali-silica plot of Mac- cannot be made definitely. Compared to the averages of donald and Katsura (1962) for the Hawaiian lavas. Engel et al. (1965), these specimens are closer to alkali The Peter I Island trachyandesites fall on the basalt than to tholeiitic basalt. On the basis of a weight trachytic side of the Daly gap identified with oceanic % of Tiθ2 in excess of 1.75, these basalts can be classi- basalt-trachyte associations, as defined by Chayes in fied as oceanic (Chayes, 1964). Tilley and Muir (1964) in terms of silica, CaO, total

354 GEOLOGY OF PETER I ISLAND

1 1 1 1 1 1 1 1 1 i i i i 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 13 - D X nephelinic "" a o αlkαlic \2 * Hawaiian © D transitional a D

11 \tholeiitic _ 0 11 (number and placement approximate a 0 ° — in high density area) A 10 α Gough Island O D A * Jones Mountains O - 9 • Mount Takahe O ° D - • Peter I Island 0 0 • — 8 Fosdick Mountains o - O L cP ° sP 0 - 0 o O O _ _ CVJ X 0 cP X. Z 6 — cQ ° — 3« Oo °o a _ V λn A δC? _ & D — * 5 x o o X O < - • — ( • 4 °‰ x c * o -b • i • - A $Á X ° o

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 35 40 45 50 55 60 65

wt. % SiO2 Figure 10. Alkalis versus silica diagram for volcanic rocks from selected districts (see text).

TABLE 6 texture and the wide variations in the estimated modes. Solidification Index (SI) of Six Rock Dunne (1946) describes xenoliths from the volcanic Specimens from Peter I Island rocks of Tristan da Cunha which have mineralogy B-2 B-3 B-4 PI-5 B-5 PI-14 similar to the Peter I Island inclusions. Lewis (1973) discusses gabbroic blocks within andesites on St. Vin- SIa 40 34.6 34.4 33 8.8 6.9 cent in the West Indies which also show mineralogical and textural similarities. In both these cases, however, MgO X 0 + Na• the plagioclases are much more calcic than in the Peter •SI» 100/MgO + Fe2< 3 2o, K2O. I Island inclusions, which show a range of Ami to An72. Most of the Peter I Island inclusions appear to be iron, and differentiation index. Chayes (1964) pointed varieties of gabbro, but some of these rocks may be out the bimodal character of Cenozoic oceanic volcanic diorite. The texture and mineralogy of these inclusions, rocks as measured by the differentiation index of along with the absence of oriented fabrics and strain Thornton and Tuttle (1960). The four basalts and two lamellae, suggest that these rocks formed at modest trachyandesites from Peter I Island conform to the depths, probably within the oceanic crust or the bimodal pattern advocated by Chayes (Figure 11). volcano. Martin and Piwinskii (1972) have argued that such a The relative abundances of the several rock types of bimodal compositional distribution is characteristic of Peter I Island are important in developing a petro- volcanic rocks formed in nonorogenic settings. genetic model. No accurate estimates are possible yet No chemical analyses were made of the gabbroid in- because of poor exposures and limited study. The clusions in the trachyandesites because of their coarse dredged specimens of Broch (1927) represent a random

355 T. W. BASTIEN, C. CRADDOCK

I6θr- o OCEANIC • CIRCUMOCEANIC A PETER I ISLAND 140- index for individual A JONES MOUNTAINS specimens MOUNT TAKAHE 120-

100 - >- o

O LU en

20-

20 30 40 50 60 70 80 90 100 THORNTON - TUTTLE INDEX After OWES ,1964

Figure 11. Thomton-Tuttle differentiation indexes (X normative Q, Or, Ab, Lc, Ne) for volcanic rocks from Peter I Island and two localities in West Antarctica, compared to frequency curves for oceanic and circumoceanic volcanic rocks (see text). sample and show these weight percentages: (1) The volcanic rocks of Peter I Island are principally basalts—94.5% (2) andesites—2.7%, (3) trachy- oceanic olivine basalts, with lesser amounts of andesite, andesites—0.9%, and (4) inclusion—0.1%. The trachy- trachyandesite, and possibly benmorite and trachyte. andesites, however, are soft and may well be under- All these rocks may represent a single volcanic series, represented in the dredged collection. In the 1960 somewhat alkaline in character, but lying mainly in the collection the weight percentages are (1) basalts—45%, transition field between the tholeiitic basalt series and (2) trachyandesites—44%, and (3) gabbroid inclu- the alkali olivine basalt series. Six chemical analyses sions—11%; but this collection was deliberately biased suggest, but do not establish, a bimodal distribution of in favor of the trachyandesites and inclusions. volcanic rocks in terms of compositions. The trachyandesites contain coarse-grained gabbro-to- SUMMARY diorite inclusions which probably formed within the Peter I Island is an oceanic volcanic island which has oceanic crust or the volcano. had its original form extensively modified by shoreline processes and glacial erosion. A prominent wave-cut platform surrounds the island; with continued erosion ACKNOWLEDGMENTS its profile will come to resemble a typical guyot. A single whole-rock K-Ar age of 12 m.y. has been ob- We appreciate the interest and counsel of the late Hisashi Kuno during a time when we were all at the University of tained on a basalt flow at Cape Ingrid. No historic vol- Minnesota. Edward Doss, then chief electrician on the U.S.S. canism has been reported, but the maximum peak Burton Island, aided in the field work on Peter I Island. This elevation measured in 1960 is considerably greater than work was supported by the National Science Foundation Of- that found by earlier visitors. fice of Antarctic Programs.

356 GEOLOGY OF PETER I ISLAND

REFERENCES Menard, H.W., 1964. Marine geology of the Pacific: New Anderson, V.H., 1960. The petrography of some rocks from York (McGraw-Hill). Marie Byrd Land, Antarctica: U.S.N.C.-I.G.Y. Antarctic Rittman, A., 1962. Volcanoes and their activity: New York Glaciol. Data 1958-59, Rept., 825-2, Part VIII, p. 1-27. (Interscience). Antarctic Report, 1964. Field report 62, Office of Antarctic Shimizu, H., 1964. Glaciological studies in West Antarctica, Programs, National Science Foundation, Washington, 1960-1962: Antarctic snow and ice studies, Am. Geophy. D.C. Un., Antarctic Res. Ser., v. 2, p. 37-64. Broch, O.A., 1927. Gesteine von der Peter I.-Insel, West Ant- Tobi, A.C., 1963. Plagioclase determination with the aid of arktis: Auh. Norske Vid. Akad., M.N.K1., no. 9, p. 1-41. the extinction angles in sections normal to (010). A critical Chayes, F., 1964. A petrographic distinction between evaluation of current albite-Carlsbad charts: Am. J. Sci., Cenozoic volcanics in and around the open oceans: J. v. 261, p. 157-167. Geophys. Res., v. 69, p. 1573-1588. Thomas, E., 1957. Chilean Antarctic expeditions, 1955-56: Craddock, C, Bastien, T.W., and Rutford, R.H., 1964. Polar Rec, v. 8, p. 525-527. Geology of the Jones Mountains area. Antarctic Geology: Thornton, C.P. and Tuttle, O.F., 1960. Chemistry of igneous Amsterdam (North Holland Publ. Co.), p. 171-187. rocks. I. differentiation index: Am. J. Sci., v. 258, p. 664- Commander Task Force Thirty Nine, Report of Operations, 684. Second Antarctic Development Project, 1947-1948, Annex Tilley, C.E. and Muir, I.D., 1964. Intermediate members of IA, p. 1-18. the oceanic basalt-trachyte association: Geol. Foren- Debenham, F. (Ed.), 1945. The Voyage of Captain ingens Stockholm Forhandl., v. 85, p. 434-443. Bellingshausen to the Antarctic Seas 1819-1821: London Tyrell, G.W., 1945. Report on rocks from West Antarctica (Hakluyt Society). and the Scotia Arc: Discovery Rept., v. 23, p. 37-102. Dubinin, A.I., 1960. Ob near Peter I Island: Inf. Bull. Soviet United States Navy Hydrographic Office, 1960. Sailing direc- Antarctic Exped., no. 23, p. 81-84. tions for Antarctica: H.O. Publ. no. 27: Washington (U.S. Dunne, J.C., 1946. Volcanology of the Tristan da Cunha Government Printing Office). Group: Norwegian Sci. Exped. to Tristan da Cunha 1937- Wilkinson, J.F.G., 1967. The petrography of basaltic rocks. 1938, Results, v. 1, p. 1-145. In Hess, H.H. and Poldervaart, A. (Eds.), Basalts, New Engel, A.E.J., Engel, C.G., and Havens, R.G., 1965. York (Interscience), v. 1, p. 163-214. Chemical characteristics of oceanic basalts and the upper Wilshire, H.C. and Binns, R.A., 1961. Basic and ultrabasic mantle: Geol. Soc. Am. Bull., v. 76, p. 719-734. xenoliths from volcanic rocks of New South Wales: J. Fenner, C.N., 1938. Olivine fourchites from Raymond Petrol., v. 2, p. 185-208. Fosdick Mountains, Antarctica: Geol. Soc. Am., Bull., v. 49, p. 367-400. Holderson, H., 1948. The "Brategg" Expedition: The Norwegian Whaling Gazette, no. 6, p. 220-227. APPENDIX Holtedahl, O., 1929. On the geology and physiography of Peter I Island Rock Specimens, Collected by Craddock some Antarctic and Sub-Antarctic islands: Norwegian at Norvegia Bay, 29 February 1960 Antarctic Exped. 1927-1928-1929, Sci. Results, no. 3, 1. Beach cobble of olivine basalt. p. 83-88. 2. Slightly vesicular basalt, from flow. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the 3. Slightly vesicular olivine basalt with inclusion of granular olivine, chemical classification of the common volcanic rocks; from flow. Canadian J. Earth Sci., v. 8, p. 523-548. 4. Vesicular, glassy olivine basalt with irridescent surface, from Kuno, H., 1962. Frequency distribution of rock types in flow. oceanic orogenic, and kratogenic volcanic associations: 5. Vesicular olivine basalt, from flow. Am. Geophy. Un., Geophys. Mono., no. 6, p. 135-139. 6. Beach cobble of olivine basalt with elongated vesicles. 7. Slightly vesicular olivine basalt, from dike 15 in. wide. -, 1968. Differentiation of basaltic magmas. In Hess, 8. Slightly vesicular olivine basalt, from flow. H.H. and Poldervaart, A. (Eds.), Basalts: New York 9. Talus clast of vesicular olivine basalt, altered to reddish-brown. (Interscience), v. 2, p. 623-688. 10. Talus clast of vesicular basalt, altered to grayish-red. Kuno, H., Yamasaki, K., Iida, C, and Nagashima, K., 1957. 11. Talus clast of vesicular basalt. Differentiation of Hawaiian magmas: Jap. J. Geol. Geog., 12. Talus clast of vesicular basalt, probably formed as bulbous v. 28, p. 179-218. squeeze-up. Le Maitre, R.W., 1962. Petrology of volcanic rocks, Gough 13. Beach cobble of porphyritic aphanite, probably trachyandesite. Island, South Atlantic: Geol. Soc. Am. Bull., v. 73, 14. Trachyandesite, from flow. p, 1309-1340. 15. Beach boulder of vesicular olivine basalt, altered to grayish-red, Lewis, J.F., 1973. Petrology of the ejected plutonic blocks of with inclusion of glassy trachyandesite. 16. Trachyandesite (?) with small gabbroid inclusion, from flow. the Soufriere volcano, St. Vincent, West Indies: J. Petrol., 17. Trachyandesite (?), from flow. v. 14, p. 81-112. 18. Beach boulder of trachyandesite (?) with gabbroid inclusions. Macdonald, G.A., 1967. Forms and structures of extrusive 19. Trachyandesite with gabbroid inclusion, from flow. basaltic rocks. In Hess, H.H. and Poldervaart, A. (Eds.), 20. Trachyandesite with gabbroid inclusions, from flow. Basalts: New York (Interscience), v. 1, p. 1-61. 21. Trachyandesite (?) with gabbroid inclusion, from flow. Macdonald, G.A. and Katsura, T., 1962. Relationship of 22. Trachyandesite (?) with gabbroid inclusion, from flow. petrographic suites in Hawaii: Am. Geophy. Un., 23. Trachyandesite (?) with gabbroid inclusion, from flow. Geophys. Mono., no. 6, p. 187-195. 24. Beach boulder of trachyandesite with gabbroid inclusion. 25. Beach cobble of trachyandesite (?) with gabbroid inclusion. , 1964. Chemical composition of Hawaiian lavas: J. 26. Beach cobble of trachyandesite (?) with gabbroid inclusion. Petrol., v. 5, p. 82-133. 27. Glassy trachyandesite (?), from flow. Martin, R.F. and Piwinskii, A.J., 1972. Magmatism and tec- 28. Trachyandesite (?) with gabbroid inclusions, from flow. tonic setting: J. Geophys. Res., v. 77, p. 4966-4975. 29. Beach cobble of trachyandesite (?) with gabbroid inclusion.

357